This invention relates to a latex of dispersed particles which may be used in an electrophoretic display. The latex is non-film-forming so that the particles may be repeatedly removed from dispersion and redispersed. The latex comprises: a) a highly fluorinated liquid solvent, and b) dispersed particles as specified herein.
U.S. Pat. No. 5,397,669 (Minnesota Mining and Manufacturing) discloses liquid toners for use with perfluorinated solvents. The patent discloses that the compositions are film-forming, allowing them to function properly as toners. (""669 at p. 8 Ins. 3-5). The ""669 patent discloses pigment particles bound to a polymer that is highly fluorinated in specific parts, and that includes monomer units having groups that bind polyvalent metal ions. The ""669 patent also discloses pigment particles bound to a polymer that is highly fluorinated in its entirety, without requiring monomers having groups that bind polyvalent metal ions.
U.S. Pat. No. 5,530,053 (Minnesota Mining and Manufacturing) also discloses liquid toners for use with perfluorinated solvents. The toners of ""053 are polymeric dyes which are highly fluorinated in specified parts and have attached chromophoric groups. The ""053 patent discloses that the toner can form a latex in perfluorinated solvent, where the toner takes a core-shell form with the hydrocarbon portion in the core and the fluorocarbon portion in the shell.
U.S. Pat. No. 5,919,293 (Hewlett-Packard) discloses ink jet inks composed of colorants in Fluorinert(trademark) solvents (Minnesota Mining and Manufacturing Co., St. Paul, Minn.), which are perfluorinated or nearly-perfluorinated alkanes.
U.S. Pat. No. 5,573,711 (Copytele) discloses the use of certain polymeric fluorosurfactants in electrophoretic image displays. The ""711 patent teaches the use of Fluorad(trademark) surfactants (Minnesota Mining and Manufacturing Co., St. Paul, Minn.), including FC-171, having the structure Rfxe2x80x94SO2N (C2H5)(CH2CH2O)nCH3, where n is about 8 and Rf is a fluorocarbon portion.
Briefly, the present invention provides a non-film-forming latex comprising a highly fluorinated liquid solvent and dispersed particles comprising a polymer comprising units according to formula I: 
wherein each (fcp) is independently selected from highly fluorinated polymer chains which may terminate at the xe2x80x94Axe2x95x90 group of another unit according to formula I so as to form a polymer molecule that contains two or more A groups linked by (fcp) groups; wherein each Q is independently selected from xe2x80x94H and straight or branched non-fluorinated polymer chains (hcp), wherein no more than one Q of each unit according to formula I may be xe2x80x94H, and wherein each (hcp) may terminate at the xe2x80x94Axe2x95x90 group of another unit according to formula I so as to form a polymer molecule that contains two or more A groups linked by (hcp) groups. The xe2x80x94Axe2x95x90 group may be the moiety according to formula II: 
wherein each R1 is independently selected from xe2x80x94H, xe2x80x94CH3, xe2x80x94F and xe2x80x94Cl; wherein each xe2x80x94R6xe2x80x94 is independently selected from divalent substituted or unsubstituted C1-C10 cyclic alkylene, or arylene groups. The (hcp) groups may be branched where polyfunctional crosslinkers are included. The (hcp) groups may contain reacting dyes as monomer units in the polymer chain. The particles may additionally contain non-reacting dyes. The particles may additionally contain charging agents.
In another aspect, the present invention provides a non-film-forming latex comprising a highly fluorinated liquid solvent and dispersed particles comprising a polymer which is the polymerization product of a mixture comprising: i) 40-99 percent by weight of one or more non-fluorinated free-radically-polymerizable monomers, ii) 1-60 percent by weight of one or more highly fluorinated macromers terminated at one or more sites with free-radically-polymerizable groups, iii) 0-35 percent by weight of one or more crosslinkers having two or more free-radically-polymerizable groups, and iv) 0-25 percent by weight of one or more reacting dyes bearing a free-radically-polymerizable group. The particles may additionally contain non-reacting dyes. The particles may additionally contain charging agents.
In another aspect, the present invention provides an electrophoretic display comprising the non-film-forming latex herein, wherein said dispersed particles may be alternately a) removed from dispersion by application of an electric field, and b) redispersed by removal or reversal of said electric field.
What has not been described in the art, and is provided by the present invention, is a non-film-forming latex of hydrocarbon/fluorocarbon particles dispersed in a fluorocarbon solvent useful in an electrophoretic display, and more specifically a latex having the small particle size and high conductance disclosed herein.
In this application:
xe2x80x9creacting dyesxe2x80x9d means dyes which are covalently bound to the polymer;
xe2x80x9cnon-reacting dyesxe2x80x9d means dyes which are not substantially incorporated into a polymer by polymerization, including every dye that is not a reacting dye;
xe2x80x9chighly fluorinatedxe2x80x9d, means containing fluorine in an amount of 40 wt % or more, but preferably 50 wt % or more and more preferably 60 wt % or more, and refers to the fluorine content of a population of chemical moieties where applicable, such as in the term, xe2x80x9cone or more highly fluorinated macromersxe2x80x9d;
xe2x80x9cnon-fluorinatedxe2x80x9d, means containing substantially no fluorine, i.e. containing fluorine in an amount of 5 wt % or less, but preferably 1 wt % or less and most preferably 0 wt %, and refers to the fluorine content of a population of chemical moieties where applicable, such as in the term, xe2x80x9cone or more non-fluorinated free-radically-polymerizable monomersxe2x80x9d;
xe2x80x9cC(number)xe2x80x9d refers to a chemical moiety containing the indicated number of carbon atoms;
xe2x80x9c(meth)acrylatexe2x80x9d means acrylate and methacrylate; and
xe2x80x9csubstitutedxe2x80x9d means, for a chemical species, substituted by conventional substituents which do not interfere with the desired product or process, e.g., substituents can be alkyl, alkoxy, aryl, phenyl, halo (F, Cl, Br, I), cyano, etc.
It is an advantage of the present invention to provide a latex of non-film-forming dye-bearing particles, which latex is useful in an electrophoretic display and can be used for repeated cycles of electrophoretically removing the particles of the latex from dispersion and returning them to dispersion.
The present invention provides a non-film-forming latex comprising a highly fluorinated liquid solvent and dispersed particles comprising a polymer comprising units according to formula I: 
wherein each (fcp) is independently selected from highly fluorinated polymer chains which may terminate at the xe2x80x94Axe2x95x90 group of another unit according to formula I so as to form a polymer molecule that contains two or more A groups linked by (fcp) groups; wherein each Q is independently selected from xe2x80x94H and straight or branched non-fluorinated polymer chains (hcp), wherein no more than one Q of each unit according to formula I may be xe2x80x94H, and wherein each (hcp) may terminate at the xe2x80x94Axe2x95x90 group of another unit according to formula I so as to form a polymer molecule that contains two or more A groups linked by (hcp) groups. Preferably some of the (hcp) groups are branched due to the inclusion of polyfunctional crosslinkers. Preferably the particles contain reacting or non-reacting dyes. Preferably the particles additionally contain charging agents.
Preferably the xe2x80x94Axe2x95x90 group is the moiety according to formula II: 
wherein each R1 is independently selected from xe2x80x94H, xe2x80x94CH3, xe2x80x94F and xe2x80x94Cl; wherein each xe2x80x94R6xe2x80x94 is independently selected from divalent substituted or unsubstituted C1-C10 alkylene, cyclic alkylene, or arylene groups. The (fcp) moieties may terminate at A groups at one or both ends. The (hcp) moieties may terminate at A groups at one or both ends, or at more than two ends if the (hcp) moiety is branched due to the inclusion of a crosslinker. Thus it is contemplated that a single molecule may contain numerous A groups linked by (hcp) groups.
The fluorocarbon polymer (fcp) portions are preferably highly fluorinated macromers. Preferred macromers include macromers of monomers selected from fluoroalkyl acrylates, fluoroalkyl methacrylates, fluoroalkyl haloacrylates and fluoroalkyl halomethacrylates. Preferably these macromers are fluoropolymer chains comprising units according to formula III: 
where each R1 is independently selected from xe2x80x94H, xe2x80x94CH3, xe2x80x94F and xe2x80x94Cl; each n is independently selected from integers from 1 to 10; and each R2 is selected independently from: highly fluorinated substituted or unsubstituted C1-C20 alkyl, cyclic alkyl, or aryl groups; xe2x80x94N(R3)SO2R4, where each xe2x80x94R3 is selected independently from xe2x80x94H and substituted or unsubstituted C1-C8 alkyl, and where each xe2x80x94R4 is selected independently from highly fluorinated substituted or unsubstituted C1-C20 alkyl, cyclic alkyl, or aryl groups. Preferably xe2x80x94R1 groups are xe2x80x94H or xe2x80x94CH3. Preferably n is 1 or 2, more preferably 1. Preferably xe2x80x94R2 is a highly fluorinated C1-C20 alkyl group, more preferably a highly fluorinated C1-C8 alkyl group. In another preferred embodiment, the xe2x80x94R2 groups of the (fcp) are a mixture selected from highly fluorinated C1-C8 alkyl groups and xe2x80x94N(R3)SO2R4, where xe2x80x94R3 is selected from C1-C8 alkyl groups xe2x80x94R4 is selected from highly fluorinated C1-C8 alkyl groups.
In another preferred embodiment, (fcp) are polyfluoroalkylethers, preferably those comprising one or more units according to the formula xe2x80x94(CF2)aCFXOxe2x80x94, where a is 0-3, but most preferably 1, and X is xe2x80x94F, xe2x80x94CF3 or xe2x80x94CF2CF3 but most preferably xe2x80x94F. More preferred polyfluoroalykylethers comprise units according to the formula xe2x80x94CF2CF2Oxe2x80x94. Preferred polyfluoroalykylethers are those according to the formula xe2x80x94NHCO2(CH2)p(CF2)qxe2x80x94Oxe2x80x94(CF2CF2O)m(CF2)r(CH2)sOH when (fcp) is monovalent or xe2x80x94NHCO2(CH2)p(CF2)qxe2x80x94Oxe2x80x94(CF2CF2O)m(CF2)r(CH2)sCO2NHxe2x80x94 when (fcp) is divalent, where p is 1-4, q is 1-5, r is 1-5, s is 1-4, m is 1-50. Preferably p is 1-2. Preferably q is 1-2. Preferably r is 1-2. Preferably s is 1-2. Preferably p is equal to s and q is equal to r. Preferably m is 5-20 and more preferably 7-15. The chain may alternately terminate with xe2x80x94F in place of xe2x80x94(CH2)sOH in the formula above.
The hydrocarbon polymer (hcp) groups are preferably non-fluorinated macromers (including co-macromers) of (meth)acrylate and other ethylenically unsaturated monomers such as styrenes. The hydrocarbon polymer (hcp) macromers are preferably polymers or copolymers of one or more of the following preferred monomers. Preferred monomers include monomers according to the formula: CH2xe2x95x90CR1xe2x80x94C(O)OR2, where xe2x80x94R1 is hydrogen or methyl and xe2x80x94R2 is selected from C1-C20 substituted or unsubstituted, straight-chain or branched or cyclic, alkyl or aryl groups. Especially preferred monomers of this group include ethyl (meth)acrylate, methyl (meth)acrylate and isobornyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, hydroxypropyl (meth)acrylate, acetoacetoxyethyl (meth)acrylate. Preferred monomers also include styrene or substituted styrene monomers such as methylstyrene.
The hydrocarbon polymer (hcp) portions preferably include non-fluorinated crosslinkers, and are therefore branched. Preferred crosslinkers include polyacrylates such as PEG Diacrylate with a molecular weight of preferably between 200 and 2000.
In order to improve resistance to film-forming, preferably the (hcp) portion of the particles contains one or more of: crosslinkers, cyclic or polycyclic monomers, aromatic monomers, and C12 or larger substituted or unsubstituted alkyl monomers. More preferably the (hcp) portion of the particles contains one or more of: crosslinkers, cyclic or polycyclic monomers and aromatic monomers.
The hydrocarbon polymer (hcp) portions may also include reacting dyes which are incorporated into the polymer chain as monomer units. Such reacting dyes contain a chromogenic portion and a reactive portion, which are not exclusive of each other. Preferably the reactive portion is a free-radically-polymerizable group, such as a vinyl or (meth)acrylate group. In one preferred embodiment, a free-radically-polymerizable group is added to a dye by derivatization with isocyanatoethyl methacrylate. In one procedure, the dye is suspended in a solvent such as FC-75 by sonication and an equivalent of isocyanatoethyl methacrylate is added dropwise, followed by two drops of dibutyltin dilaurate catalyst and continued sonication and agitation for 1 hour. A preferred reacting dye and method of preparation is disclosed in the examples below.
The dispersed particles may also include non-reacting dyes. The non-reacting dyes have greater affinity for the particles than for the solvent and are therefore contained in the particles. Preferably the non-reacting dyes have greater affinity for the (hcp) portions of the particle than for the (fcp) portions or the solvent. Preferred non-reacting dyes are disclosed in the examples below. The particles according to the present invention preferably contain no particulate pigments.
Without wishing to be bound by theory, it is believed that the particles take the form of a hydrocarbon-polymer-rich core and a fluoropolymer-rich shell. It is believed that most dyes are incorporated in the core due to compatibility with the hydrocarbon material of the core. Thus it is believed that reducing the fluoropolymer content of the particles improves the optical properties of the particle by allowing easier access to the dyed core. In addition, it is believed that reducing the fluoropolymer content of the particles can improve the particle""s resistance to film formation. Preferably the particles are composed of 40-99 percent by weight non-fluorinated hydrocarbon polymer and 1-60 percent by weight highly fluorinated fluoropolymer. More preferably the particles are composed of 60-99 percent by weight non-fluorinated hydrocarbon polymer. More preferably the particles are composed of 1-40 percent by weight highly fluorinated fluoropolymer. Most preferably the particles are composed of 1-10 percent by weight highly fluorinated fluoropolymer. However, greater fluoropolymer content is acceptable as particle size is decreased. In particles of less than 200 nm average diameter, the particles are preferably composed of 10-40 percent by weight highly fluorinated fluoropolymer, more preferably 10-25 percent by weight highly fluorinated fluoropolymer.
The dispersed particles may also include charging agents. The charging agent renders the particle mobile under the influence of an electric field. In addition, the charge imparted to the particles by the charging agent creates an electrostatic repulsion between particles which improves resistance to film formation. Like non-reacting dyes, the charging agent has a greater affinity for the particles than for the solvent and is therefore contained in the particles. Preferably the charging agent has a greater affinity for the (hcp) portions of the particle than for the (fcp) portions or the solvent. The charging agent is preferably cationic, more preferably a quaternary ammonium cation. Preferred charging agents include 1-ethyl-3-methyl-1H-imidazolium bis(trifluoromethylsulfonylamide), which may be prepared as disclosed in the examples below; (C4H9)3N:HOC(O)xe2x80x94C7F15; (C3H7)4N+xe2x88x92OC(O)xe2x80x94C7F15; C4H9)4N+xe2x88x92OC(O)xe2x80x94C9F19; C7F15xe2x80x94CO2H; and combinations thereof.
Latexes according to the present invention preferably demonstrate a high conductance as measured by the method described in the examples below. Measured conductance is taken to reflect the charge/mass ratio (charge density) of the particles in suspension, whether imparted by the charging agent or inherent in the particle itself. Preferred latexes according to the present invention have a conductance of 1 picomho/cm or more, more preferably 4 picomho/cm or more, and most preferably 9 picomho/cm or more. However, lower conductance is acceptable when particle size is decreased. In particles of less than 200 nm average diameter, conductance is preferably 0.1 picomho/cm or more.
The average diameter (particle size) for the dispersed particles of the latex is preferably measured by the method described in the examples below. Smaller particles are preferred for a number of reasons, including greater and faster mobility and lesser tendency to form a film. Preferably the particles have an average diameter of 1000 nm or less, more preferably 350 nm or less, more preferably 300 nm or less, more preferably 250 nm or less, and most preferably 200 nm or less. The seed method of latex formation described below has been found to produce exceptionally fine particles. That method and the resulting fine particles are preferred.
The solvent may be any suitable highly fluorinated solvent. The solvent is preferably a fluorocarbon, especially a branched or unbranched, cyclic or non-cyclic fluoroalkane. Preferred solvents include FLUORINERT(trademark) fluorinated solvents available from 3M Company, St. Paul, Minn. Two especially preferred solvents are FLUORINERT FC-75, a perfluorinated C8 solvent, CAS No. [86508-42-1], and FLUORINERT FC-84, a perfluorinated C7 solvent, CAS No. [86508-42-1].
The density of particles in solvent (solids content) may be any level at which the dispersion is stable and does not significantly coagulate. For use of the latex in an electrophoretic display, the solids content may be any level that allows proper functioning over repeated cycles. Preferably, the solids content is less than 10 wt %, more preferably less than 5 wt %, and most preferably less than 2 wt %.
The latexes according to the present invention may be incorporated into electrophoretic displays. A typical display comprises two planar electrodes defining a thin gap between them which holds the latex. When a sufficient voltage of the correct polarity is applied, the suspended particles are drawn out of suspension and onto one electrode. That electrode, which is substantially transparent, forms the inner surface of a viewing glass, such that the particles form an image viewed through the glass. In contradiction to the characteristics of an electrostatic toner, which must form a permanent image under analogous conditions, the latex of the present invention must return to suspension when the voltage is removed or reversed.
The latexes of the present invention have high resistance to film formation when used in electrophoretic display devices. To determine resistance to film formation, an actual device may be used or a breadboard device as described in the examples below. Latexes of any solids content may be tested but preferably the solids content is 1 wt %. The device is preferably used in a normal manner, alternately applying and removing (or reversing) the typical use voltage. The voltage should be sufficient to remove particles from suspension and create an image when applied. Preferably the latexes are non-film-forming to the extent that they redisperse completely (by appearance to the naked eye) after at least twenty cycles, more preferably after at least 100 cycles, and most preferably after at least 10,000 cycles. Without wishing to be bound by theory, it is believed that the resistance to film formation demonstrated by the latexes of the present invention is aided by incorporation of crosslinkers, by the choice of high Tg monomers such as 3,4 methyl styrene, isobornyl acrylate, by reduction of the fluoropolymer content resulting in a thinner outer shell zone, by increasing the electrostatic charge (conductance) of the particles, by exclusion of particulate pigments, which may be replaced with dyes. The latexes and particles according to the present invention are preferably non-film-forming due to one or more of the preceding conditions.
Any suitable method of synthesis which results in the latexes according to the present invention may be used. One method of making latexes according to the present invention is illustrated by the syntheses of latexes L1 to L15 in the Examples below. In this method, a highly fluorinated macromer is synthesized by polymerization and the macromer is then derivatized to add a terminal polymerizable group. The macromer is then polymerized together with non-fluorinated monomers, preferably using a polymerization initiator and optionally a crosslinker, to form the latex particle. Reacting dyes must be added prior to polymerization. Non-reacting dyes and charging agents may be added at any stage, but are preferably added prior to polymerization. All reactions preferably take place in a highly fluorinated solvent. Preferably the reaction mixture is no more concentrated than 15 wt % reactants relative to the amount of solvent.
A more preferred method of making the latexes of the present invention is designated the seed method, and is illustrated by the syntheses of latexes L16 to L18 in the Examples below. In this method the polymerization to form the latex particles is performed in two steps. First, the highly fluorinated macromer and a fraction of the non-fluorinated monomers are polymerized with agitation to form a population of seed particles. The weight ratio of highly fluorinated macromer to non-fluorinated monomer is preferably between 1:2 and 9:1. In the second step, the remainder of the non-fluorinated monomers are added. The amount of additional monomer is preferably at least 10% of the weight of the seed particles, and preferably not more than 20 times the weight of the seed particles. Preferably the reaction mixture is no more concentrated than 15 wt % reactants relative to the amount of solvent. The seed method can achieve a smaller average particle diameter, preferably 250 nm or less, and more preferably 200 nm or less.
This invention is useful in electrophoretic image displays.